What we called TrustScore or whatever you want to call it does not include the engagement, at least how we measure it, and follower/following counts.
The mosts important paramaters are mostly historical, there’s node for the creation stage (CS), where IG assigns an score for each item, such as: where the account was created, APP vs Web, IP, Proxified?, email quality, PVA, geo location, wifi, 4G/LTE, Device brand, model, OS version etc.
This part of the score stays with the account for ever.
Then you have the variable scores only related with actions performed by the account, here IG also gives different values to the actions, taking into consideration where they were peformed, how often, etc. (Spam detections systems are used here, IG anti-spam systems are the best for far, but they only “inform” other systems of the activities and don’t take any action.).
The more spammy the action the lower the score it gets, basically when you cross certain threshold IG penalizes the total score of that particular category, that also affects the score of the Actions category.
So imagine the number going UP when you do a few likes, saves. comments etc and then! you start to DM like crazy and the number goes down.
Paterns are analyzed constantly to compare you with simimlar accounts and also with your previous behaviours.
All accounts are born with a cap let’s say 100 units, every time the account break any rule units are deducted, you can go even negative. The question that everybody ask is if you can recover units, the answer is YES but only happens with actions that IG consider good signals. Why some accounts never come back? because they keep repeating the same mistakes again and again.
I could keep giving details, is a fascinating topic, but I prefer to answer any question you may have.
Note: if you wonder where the info is coming from, we have periodic meetings with FB/IG engineers in our office.